1. Field of the Invention
The present invention relates in general to a backplane for interconnecting printed circuit boards and in particular to a backplane providing programmably adjustable routing and active buffering of signals passing between the boards.
2. Description of Related Art
Electronic systems such as personal computers typically include several printed circuit boards (PCBs) which communicate with one another through a backplane. The backplane, usually implemented on a mother board containing slots for receiving the PCBs, includes conductive traces on the motherboard forming shared bus and/or point-to-point connections between the slots. Most backplane designs are passive in that they do not actively buffer (amplify) signals passing between the PCBs.
Designing a passive shared bus backplane that can support data transfer rates of 40 MHz or higher is difficult. FIG. 1A illustrates various devices that may be connected to a line 1 of a shared bus. FIG. 1B is an equivalent impedance diagram of FIG. 1A. As illustrated in FIGS. 1A and 1B, a line 1 of a shared bus may be connected to a large number of drivers 2, receivers 3 or transceivers 4, each contributing a capacitive load of approximately 5 pf, 10-12 pf and 15-20 pf, respectively. In addition the printed trace on the motherboard that form line 1 contributes another 1.5 pf/inch. The potentially large loading of a bus line, often adding up to 250-300 pf, can result in a delay of 12-15 ns and can limit the performance of a shared bus backplane. Therefore for a given performance, passive backplanes can serve only a limited number of components and the loading provided by each component must be carefully controlled.
The shared bus backplane has an advantage in that the particular slot in which a PCB is installed is irrelevant; each slot provides the same access to the backplane. However in applications where high speed communication between particular PCBs is needed, backplanes provide point-to-point connections between the PCBs. In such applications a backplane provides a connection from a driver on one PCB to receivers on one or more other PCBs but not to receivers on every PCB. The interconnections between slots is fixed by the arrangement of conductors on the motherboard interconnecting the slots. Therefore the slot in which a PCB is inserted determines how the PCB communicates with the rest of the system. Any change in the routing of signals between PCBs requires a change in the motherboard conductor arrangement.
What is needed is a backplane architecture for routing signals between PCBs, wherein common nodes have low and uniform impedance regardless of the number of components connected thereto, and wherein routing of interconnections between PCBs its easily modified.